The present invention relates to a magnet suitable for use in a voice coil motor used in the external memory of an electronic computer or for a motor used in household electrical appliances or factory automation (FA) devices, and, more particularly, to a magnet having a complicated shape which is very difficult to fabricate using known techniques.
As reduction in size, high-speed operation, and increase in the capacity have been demanded in the external memories of electronic computers, in household electrical appliances and in factory automation (FA) devices, there is an increasing demand for a magnet which has more excellent magnetic characteristics as a component of these electrical devices. This is because any space accommodating a motor in these electrical devices is limited, which in turn limits the shape of the motor to a thin and flat one, and hence the shape and characteristics of a magnet are demanded to be thin, flat and highly efficient.
Further, magnets having various shapes that can meet various applications have been demanded. For example, voice coil motors (hereinafter referred to as VCM) used in the external memories of electronic computers are sorted into two kinds by shape: one employs a ring-shaped magnet, and the other uses flat and plate-like magnets, as shown in FIGS. 2 and 3. Since the electromagnetic force is proportional to Bg. le, where Bg is the magnetic flux density of a gap and le is the effective length of a coil, larger and flatter plate-shaped magnets or longer ring-shaped magnets having a larger diameter have been sought after so that better magnetic characteristics and a large le can be provided. Under these circumstances, attempts have been made to employ a rare-earth element (R) - Fe - B type permanent magnet having both larger residual flux density (hereinafter referred to as Br) and larger inherent coercive force (hereinafter referred to as iHc) in the VCM in place of a conventionally employed SmCo magnet. The Japanese Patent Laid-Open No. 61-266056 discloses a magnet for use in the VCM which has a magnetic flux density Bg in a magnetic circuit increased in order to improve an electromagnetic force (k.sub.F), to increase the control gain (related to 1/k.sub.F) for a transfer function, and to decrease the positioning errors. Japanese Patent Laid-Open No. 61-210862 discloses a magnet for use in the VCM which has a magnetic flux density of not less than 9000 G at a operating point and an excellent rectangularity ratio in the demagnetizing curve.
Disk rotor type brushless motors employed in household electrical appliances such as VTR and cassette decks incorporate a disk-shaped rotor made of a permanent magnet such as that shown in FIG. 4. In this kind of motor, a decreased moment of inertia reduces the ability to cope with disturbances and increases in wow and flutter (unevenness in rotation). Therefore, flat and large disk-shaped magnets have been sought in order to improve the "flywheel effect" (GD.sup.2), where D is the diameter of the rotor and G is the gravitational constant.
The present inventor proposes a concept of "surface magnification" to standardize the shape of magnet for use in a motor. More specifically, the surface magnification is defined by (the volume of a magnet)/(the thickness in the direction of easy magnetization).sup.3, and it can be said that permanent magnets having a large surface magnification have been demanded when improving performance of office automation (OA) devices.
Conventionally, permanent magnets are manufactured by the transverse magnetic field compacting method, in which the direction of application of a magnetic field is perpendicular to the direction of compacting. They are also manufactured by the longitudinal magnetic field compacting method in which the direction of application of the magnetic field is parallel to the direction of compacting. It is known from experience that rare earth type magnets made of the same materials have different magnetic characteristics when the different methods are adapted to manufacture the magnets, and that the former method produces a permanent magnet having a higher Br.
However, it is very difficult to adapt the transverse magnetic field compacting method to the manufacture of permanent magnets having shapes such as those shown in FIGS. 5(a) to 5(c). The magnet shown in FIG. 5(a) has a ring-like shape in which the direction of easy magnetization (M) coincides with the direction of thickness. The magnet shown in FIG. 5(b) has a C-shape in which L is long compared with the width W. FIG. 5(c) shows a magnet having a fan-like shape in which L is long.
Therefore, magnets for use in VCM and magnets for use in disk-rotor type brushless motors must be manufactured by the longitudinal magnetic field compacting method. However, since the direction of application of the magnetic field coincides with the direction of compression in this method, the orientation of grains are disturbed in the direction of thickness, decreasing Br during the compacting. Therefore, it has been impossible to manufacture anisotropic sintered magnets appropriate for the use which have the above described shapes.
In the transverse magnetic field compacting method, in which the direction of compression in the sintered magnets is perpendicular to the direction of application of the magnetic field, it is very difficult to provide uniform orientation of grains throughout a sintered magnet having a surface magnification of 6 or above (manufactured by powder metallurgy) due to nonuniform compression pressure distribution. Further, there is such shortcoming that a large-scale magnetic field application means must be used to obtain the magnet. Therefore, it has been impossible to manufacture a sintered permanent magnet having a surface magnification of 6 or more.
In order to lower the temperature coefficient .alpha.(%/.degree. C.) of Br in the sintered magnet, the amount of Co added is increased. However, this addition rapidly decreases iHc.
Accordingly, warm working magnets have been proposed in order to eliminate the above-described problems of the sintering (see European Patent Laid-Open Publication No. 0133758). In warm working, magnetically anisotropic magnets are manufactured by plastically warm deforming the alloy powder produced by the rapid quenching method. In this working, since the direction of compression becomes substantially identical with the direction of easy magnetization during plastic deformation, disturbance of orientation of grains only occurs at a very low level, unlike the sintered magnets, and this makes warm working suitable to the manufacture of plate-like magnets.
However, in this manufacturing method utilizing plastic deformation, heating at a temperature of 700.degree. C. or above is limited to 5 minutes or less in order to prevent reduction in the coercive force (iHc) which reduction is caused by the growth of crystal grains. Therefore, it is difficult to manufacture a magnet for a motor which magnet is formed by uniformly heating a large compact body and which magnet has a surface magnification of 6 or above.
Further, there is such problem that, even if a magnet is manufactured by heating a compact body for less than 5 minutes, it has a coercive force of 12 KOe at most and is inferior in heat-resistivity.